The present invention relates in general to waveguide bandpass filters, and pertains, more particularly, to a low loss microwave bandpass filter having high attenuation, not only at adjacent stop band frequencies, but also at harmonics of the pass band. The filter of this invention exhibits good rejection of the second harmonic of the pass band, all incorporated in a single filter structure, thus not requiring the usual cascading of a bandpass filter and a low pass filter.
Waveguide bandpass filters may presently be considered as falling into two main categories. One filter is an iris or post-coupled filter and the other is an evanescent mode waveguide filter. In the first category, there are provided a series of tuned waveguide cavities coupled with irises or posts across the waveguide. These filters require good electrical contact between the coupling iris or post in both the top and bottom walls of the waveguide. Such filters usually use waveguide of the same cross-sectional dimension as the transmission waveguide for the band required. Cavities having a fairly high unloaded Q factor are possible, enabling low loss narrow band filters to be constructed. However, the half wavelength rectangular cavities become one wavelength resonant cavities in the HO2 mode near the second harmonic of the desired pass band. Also, most simple coupling iris structures permit the free transmission of the second harmonic in the HO2, HO3, or Eln modes. Thus, it is practically impossible to construct a bandpass filter, particularly in a standard waveguide (in a standard waveguide, the second harmonic of the required frequency has at least five different modes by which it propagates down the waveguide) which controls the second harmonic rejection, without designing a separate low pass filter and cascading the two.
The second category of present bandpass filters is the evanescent mode waveguide filter which comprises a series of evanescent waveguide sections, each resonated by capacitive elements at their section junctions. Typically, these waveguide filters are based upon a filter design having capacitive gapped posts across a reduced dimension waveguide. With these filters, the second harmonic may only be transmitted in a single (HO1) mode so that any spurious pass bands are sited away from this frequency. The resonators so constructed in this evanescent mode filter have a lower Q and their frequency is dependent on a thin gap at the end of the post. Thus, these filters are limited in use to broad band applications. The much lower Q factor of the resonant circuits formed in this way leads to high pass band loss in narrow band filters. Also, in this waveguide filter, the capacitive gaps (gapped posts) are very critical to set up and are temperature dependent.
Accordingly, it is an object of the present invention to provide an improved waveguide bandpass filter that combines low loss bandpass filter characteristics with high attenuation at harmonic frequencies.
Another object of the present invention is to provide an improved waveguide bandpass filter which is characterized by good rejection of the second harmonic of the pass band, all incorporated into a single filter structure. As indicated previously, the filter of the present invention eliminates the need for cascading filters and eliminates the need for, in particular, cascading a bandpass and low pass filter.
A further object of the present invention is to provide an improved waveguide bandpass filter that provides more predictable performance, particularly at harmonics, by reducing over-moding effects.
Still another object of the present invention is to provide an improved waveguide bandpass filter that is constructed so as to eliminate the need for employing a separate harmonic filter in association therewith.
Still another object of the present invention is to provide an improved waveguide bandpass filter that has high rejection of harmonic frequencies and that may be constructed in a convenient mechanical arrangement, readily incorporated into a waveguide integrated package.